Although the invention is generally applicable to all types of such cutting tools, which are used for chip removing machining of, above all, workpieces of metal, the same is in practice based on problems, which are particularly difficult to master in connection with the manufacture of drills or drill bodies for hole-making purposes.
The cutting edges of drills requisite for the chip removal may either be permanently integrated in the drill body or be included in replaceable cutting inserts of a particularly hard material, such as cemented carbide. In order to evacuate the removed chips from the recessed hole, conventional short hole drills are formed with external chip flutes (usually two in number), which extend rearward from the cutting inserts or the cutting edges of the front tip of the drill toward the rear end. The chip flutes are delimited between bars, inside which ducts are formed for feed of liquid to the front end of the drill with the purpose of cooling and lubricating the cutting inserts. Because the chip flutes are deep and mutually spaced-apart by fairly thin webs in the center of the drill, the internal liquid ducts have to be located as near the envelope surface of the bars as possible, where the material is sufficiently sturdy to house the same.
A popular type of twist drill (i.e., a drill, the external chip flutes and bars of which extend helicoidally along the front part of the drill), which during operation is fixedly mounted in order to machine a rotating workpiece, is formed with two inlets to the internal liquid conduits, viz. a rear inlet, which mouths in the rear end surface of the drill body, and a side inlet, which mouths in the envelope surface, usually in a collar, which separates a front part of the drill from a rear part. In such a way, the user is offered the possibility of—depending on the existing machine equipment—engage the threaded hose or conduit connections of the machine to either the side inlet or the rear inlet, the unutilized inlet being closed by means of a likewise threaded plug. However, if a liquid-leading duct inside a tool body would be crossed or intersected by a threaded bore, an imminent risk of liquid leakage arises via the thread. For this reason, it has previously been necessary to manufacture drills of the type in question by assembling two parts, viz. a front part in which the bars and the liquid ducts are formed, as well as a rear part, in which the two inlets are formed and interconnected via a centrally running duct, which mouths in a hollow space with which the two liquid ducts of the front part can communicate after assembly of the two parts. The manufacture of the front part is carried out by drilling, in a cylindrical blank of steel, straight holes in order to form the two liquid ducts, preferably by pipe-drilling, and then by hot working turn the blank so that the ducts obtain a helix shape having the desired pitch. After that, grooves are milled having the same helix shape as the ducts, in order to form chip flutes. In a concluding step, the two parts are integrated with each other via a suitable metallurgical joint, e.g., a welded joint.
However, a disadvantage of twist drills, which are manufactured in the above-mentioned way, is that the completed drill may obtain inferior precision and varying tolerances, which leads to poor machining results. Therefore, in practice, it occurs too often that the finish-produced drills have to be discarded.
An entirely different type of twist drill, which rotate in order to machine fixedly mounted workpieces, lack side inlets, and are formed with liquid ducts, which mouth directly in the rear end of the drill body without having any threads. In such a way, the drill body can be manufactured in a solid design based on a blank, in which the two holes of the liquid duct can be pipe-drilled all the way between the front end and the rear one, after which the front part of the blank on one hand is turned in a heated state in order to give the ducts the desired helix shape, and on the other hand is machined by milling in order to form the external chip flutes. As a consequence of the fact that such drills can be manufactured from one single blank in a continuous piece, the completed drill obtains considerably better tolerances and performance than the previously described, two-part drill.
The present invention aims at obviating the respective disadvantages of previously known drill bodies or basic bodies and at providing an improved basic body of the tool.
An object of the invention to provide a basic body made with one or more liquid conduit ducts, in which the requisite inlet or inlets to the ducts optionally can be placed on suitable spots along the same without risk of liquid leakage between, on one hand, the female thread of the inlet, and on the other hand the male thread of a hose connection or a plug. It is also an object to provide a basic body having improved flexibility relating to the possibilities of connecting to the same different hose connections.
Another object is to provide exactly a drill body, which can be manufactured in one single solid piece and be formed with one or more inlets, which are threaded and in spite of this ensuring leakage-proof connection of hoses or plugs. Within the field of drills, an embodiment of the invention particularly aims at providing an improved twist drill having double, internal liquid feeding ducts and having optimal properties in respect of dimensional accuracy and performance.
Yet another object of the invention is to provide a basic body, which can be manufactured of a material, e.g., a composite material, which, in contrast to steel, is hard to form with threads.